DOC-984: Update Serp API credentials to template

[freakwriter]

Update to use numbered steps and a direct link to the relevant page.

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[github-actions[bot]]

Overall readability score: 42.26 (🟢 +0.01)

File	Readability
serp.md	29.78 (🟢 +9.73)

View detailed metrics

🟢 - Shows an _increase_ in readability 🔴 - Shows a _decrease_ in readability File | Readability | FRE | GF | ARI | CLI | DCRS --- | --- | --- | --- | --- | --- | --- [serp.md](https://github.com/n8n-io/n8n-docs/blob/68056270b8919a0e18598c100bf4951a3ef3c67f/docs/integrations/builtin/credentials/serp.md "docs/integrations/builtin/credentials/serp.md") | 29.78 | 18.52 | 10.55 | 18.3 | 18.88 | 8.92   | 🟢 +9.73 | 🟢 +7.14 | 🟢 +0.96 | 🟢 +2.4 | 🟢 +0.12 | 🟢 +1.01 Averages:   | Readability | FRE | GF | ARI | CLI | DCRS --- | --- | --- | --- | --- | --- | --- Average | 42.26 | 34.17 | 11.79 | 15.03 | 14.46 | 8.6   | 🟢 +0.01 | 🟢 +0.01 | 🟢 +0 | 🟢 +0 | 🟢 +0 | 🟢 +0

View metric targets

Metric | Range | Ideal score --- | --- | --- Flesch Reading Ease | 100 (very easy read) to 0 (extremely difficult read) | 60 Gunning Fog | 6 (very easy read) to 17 (extremely difficult read) | 8 or less Auto. Read. Index | 6 (very easy read) to 14 (extremely difficult read) | 8 or less Coleman Liau Index | 6 (very easy read) to 17 (extremely difficult read) | 8 or less Dale-Chall Readability | 4.9 (very easy read) to 9.9 (extremely difficult read) | 6.9 or less

[freakwriter]

@ayatnkw this one's a very simple credential so should be a fast review!